The GnRH-R is a key mediator in the integration of the neural and endocrine systems. Normal reproduction depends on the pulsatile release of physiological concentrations of GnRH which binds to specific high affinity pituitary receptors and triggers the secretion of the gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH). Whereas physiological concentrations of GnRH orchestrate normal reproduction, high levels of agonist lead to an opposite response, the suppression of gonadotropin secretion. The capacity of GnRH analogues both to activate and to inhibit the hypothalamic-pituitary-gonadal axis has led to their wide clinical utility in the treatment of a variety of disorders ranging from infertility to prostatic carcinoma.
The responsiveness and capacity of the gonadotrope GnRH-R is influenced by agonist, concentration and pattern of exposure (Clayton, 1989, J Endocrinol 120: 11-19). Both in vivo and in vitro studies have demonstrated that low concentration pulsatile GnRH is trophic to the receptor and that a high concentration of agonist induces receptor down-regulation and desensitization. The binding of GnRH to its receptor stimulates phospholipase C and generates inositol-1,4,5-triphosphate and diacylglycerol (Huckle & Conn, 1988, Endocrine Reviews 9: 387-395). These second messengers, in turn, release calcium from intracellular stores and activate protein kinase C. Receptor up-regulation appears to involve both protein kinase C and calcium (Huckle & Conn, 1988, Endocrine Reviews 9: 387-395; Huckle et al., 1988, Journal of Biological Chemistry 263: 3296-3302; Young et al., 1985, Journal of Endocrinology 107: 49-56). It is not certain which effectors underlie down-regulation.
While great progress has been made in understanding the mechanisms underlying GnRH-R regulation and desensitization through receptor is binding studies, direct measurement of GnRH-R gene transcription and biosynthesis has not been possible. Cloning of the GnRH-R cDNA would advance the evaluation of GnRH-R activation, regulation and uncoupling. Determining the primary sequence of the receptor would facilitate the directed design of improved analogues. However, despite intensive interest, heretofore, the GnRH-R gene has not been cloned and expressed in any species.